Changes Around Us: Physical and Chemical – Short Answer Type Questions
Class 7
Science — Chapter 5: Changes Around Us (Physical & Chemical)
NCERT-aligned Short Answer Questions — Clear and concise answers for CBSE Class 7 exam practice.
CBSE Board Examination
Focus: identifying types of changes, experimental observations, reversibility and examples.
50 Short Answer Type Questions & Answers
Definitions & Basics (1–8)
- Q1: Define a physical change.A: A change in which the form or state of a substance changes but its chemical composition remains unchanged.
- Q2: Define a chemical change.A: A transformation where substances react to form one or more new substances with different properties.
- Q3: Give two everyday examples of physical changes.A: Melting of ice; tearing of paper.
- Q4: Give two everyday examples of chemical changes.A: Rusting of iron; cooking an egg.
- Q5: What is meant by reversible change?A: A change that can be undone by simple physical means (e.g., water ⇄ ice).
- Q6: What is meant by irreversible change?A: A change where original substances cannot be easily recovered (e.g., burning paper).
- Q7: Is dissolving sugar in water physical or chemical? Why?A: Physical — sugar molecules remain chemically the same and can be recovered by evaporation.
- Q8: Is formation of curd from milk physical or chemical? Explain.A: Chemical — new substances form due to action of bacteria/enzymes, changing properties and taste.
Signs of Chemical Change (9–16)
- Q9: List four common signs of a chemical change.A: Colour change, gas evolution, formation of precipitate, temperature change (heat released/absorbed).
- Q10: Why is evolution of gas considered a sign of chemical change?A: Because gas production indicates new substances are being formed during reaction.
- Q11: Give an example where heat is released in a chemical change.A: Combustion of wood — heat and light are released.
- Q12: Can a chemical change occur without any obvious sign? Explain.A: Yes — some reactions show subtle changes (e.g., slow oxidation) requiring tests or time to observe.
- Q13: How does formation of a precipitate indicate chemical change?A: Two clear solutions forming an insoluble solid means new substance has formed, signifying a reaction.
- Q14: Is change of smell always a sign of chemical change?A: Often yes, because new volatile substances may form; but consider other evidence too.
- Q15: What simple test confirms hydrogen gas?A: The 'pop' test with a burning splint — hydrogen burns with a pop sound.
- Q16: Why is colour change alone not conclusive proof of a chemical change?A: Colour can change due to physical factors (concentration, phase); corroborate with other signs.
Reversibility & Comparisons (17–24)
- Q17: Give an example of a reversible chemical change (if any).A: Esterification and hydrolysis can be reversible under controlled lab conditions, but rare in daily life.
- Q18: How would you distinguish melting of wax from burning of wax?A: Melting is physical — wax returns on cooling; burning produces new substances, smoke, heat and cannot be reversed.
- Q19: Is rusting reversible? Explain briefly.A: Practically irreversible — rust removal is possible but restoring original metal requires reduction processes, not simple reversal.
- Q20: Which is faster: physical change or chemical change? Explain with example.A: It depends: some physical changes (melting) are quick; some chemical changes (combustion) can be very fast, while others (rusting) are slow.
- Q21: Is evaporation of alcohol physical or chemical? Why?A: Physical — only state changes; chemical identity remains same.
- Q22: Compare cutting paper and burning paper.A: Cutting is physical (shape change); burning is chemical (ash, heat, new substances form).
- Q23: Is dissolving salt in water reversible? How to prove?A: Yes — evaporate water to recover salt; demonstrates physical change.
- Q24: Why are many chemical changes irreversible in daily life?A: Because they produce new stable substances or use energy that is not easily reversed without special processes.
Short Experiments & Observations (25–32)
- Q25: What is observed when magnesium ribbon burns?A: Bright white flame and a white powder (magnesium oxide) — chemical change.
- Q26: What happens when dilute HCl reacts with zinc?A: Effervescence due to hydrogen gas and zinc dissolves to form salt — chemical reaction.
- Q27: Describe a test to show formation of precipitate.A: Mix barium chloride and sodium sulfate solutions; a white precipitate of barium sulfate forms.
- Q28: What observation shows an exothermic reaction?A: The temperature of the surroundings rises and heat is released.
- Q29: How does one detect carbon dioxide gas in lab experiments?A: Bubble gas through limewater — it turns milky, indicating CO₂.
- Q30: What is a common sign of combustion?A: Emission of heat and light and formation of new products like ash or CO₂.
- Q31: What observation suggests a reaction is endothermic?A: The surroundings become colder; reaction absorbs heat.
- Q32: Why is vinegar and baking soda reaction used in class demonstrations?A: Because it produces brisk effervescence (CO₂), easy and safe to observe gas evolution.
Daily‑life Applications (33–40)
- Q33: Give an example of chemical change in the kitchen.A: Cooking rice turns starch into softer edible form — chemical changes occur during heating.
- Q34: Give an example of physical change in the kitchen.A: Melting butter while frying—state change reversible on cooling.
- Q35: How does rusting affect iron tools and structures?A: It weakens them by converting metal into flaky oxides, causing structural failure over time.
- Q36: Name two methods to prevent rusting.A: Painting and galvanisation (zinc coating) are common methods.
- Q37: Why is food digestion classified as chemical change?A: Enzymes break complex molecules into simpler ones, forming new chemical substances absorbed by the body.
- Q38: Is milk turning into curd a physical or chemical change? Why?A: Chemical — lactic acid bacteria change milk proteins and form curd with different properties.
- Q39: How does baking powder make cakes rise (chemical context)?A: It releases CO₂ gas on heating/with moisture, causing batter to expand — chemical decomposition reaction.
- Q40: Why should we use multiple signs to identify a chemical change in daily life?A: Because some physical processes mimic single signs (e.g., bubbling), so multiple evidences make conclusion reliable.
Concept Checks & Common Pitfalls (41–46)
- Q41: Is tearing a cloth a chemical change? Explain.A: No — tearing is physical; cloth composition remains same.
- Q42: Does boiling water change its chemical composition?A: No — boiling is a physical change; H₂O molecules remain unchanged.
- Q43: Is bleaching of clothes a physical or chemical change?A: Chemical — bleaching involves chemical reactions that alter dyes and colours.
- Q44: Why might some students confuse dissolving with chemical change?A: Because appearance changes, but dissolving usually doesn't form new substances; test by evaporation.
- Q45: Can heating be both physical and chemical? Give example.A: Yes — heating ice (physical) and heating sugar until it chars (chemical).
- Q46: Is making tea physical or chemical? Briefly justify.A: Mostly physical (infusion) but includes some chemical changes (extraction and mild chemical reactions altering flavour compounds).
Exam Strategy & Quick Tips (47–50)
- Q47: How to structure a short 2–3 line answer identifying a change?A: State type (physical/chemical), list 1–2 observed signs, and conclude with reasoned statement.
- Q48: What one-line definition of chemical change should you remember for exams?A: A chemical change produces one or more new substances with different properties from the reactants.
- Q49: Suggest a quick classroom demonstration to show gas evolution.A: Mix vinegar and baking soda in a container to show brisk bubbling of CO₂.
- Q50: Give a short tip to avoid mistakes while answering change-related questions.A: Always mention observations (signs) and link them to your conclusion — this scores marks and shows understanding.